1. Field of the Invention
The present invention relates to a fundus imaging method, a fundus imaging apparatus, and a storage medium.
2. Description of the Related Art
Recently, as an ophthalmic imaging apparatus, an SLO (Scanning Laser Ophthalmoscope) has been developed, which two-dimensionally irradiates the fundus with a laser beam, receives reflected light, and images the light. In addition, as an ophthalmic imaging apparatus, an imaging apparatus using low-coherence light interference has been developed. The imaging apparatus using lower-coherence light interference is called an OCT (Optical Coherence Tomography), which is used for the purpose of obtaining a tomogram of the fundus or its neighboring region, in particular. Various types of OCTs have been developed, including a TD-OCT (Time Domain OCT) and an SD-OCT (Spectral Domain OCT). Ophthalmic imaging apparatuses have recently been increased in resolution with increasing NA of irradiation lasers.
When, however, imaging the fundus, it is necessary to perform imaging through optical tissues of the eye, such as the cornea and the crystalline lens. For this reason, with an increase in resolution, the aberrations of the cornea and crystalline lens have started to greatly influence the image quality of captured images.
Under the circumstances, studies have been made on an AO (Adaptive Optics)-SLO and AO-OCT, in which an optical system incorporates an AO function for measuring the aberrations of the eye and correcting them. For example, non-patent literature 1 (Y. Zhang et al, Optics Express, Vol. 14, No. 10, 15 May 2006) discloses an example of an AO-OCT. Such AO-SLO and AO-OCT measure the wavefront of the eye by the Shack-Hartmann wavefront sensor system. The Shack-Hartmann wavefront sensor system is designed to measure the wavefront of the eye by applying measurement light to the eye and making a CCD camera receive the reflected light through a microlens array. An AO-SLO or AO-OCT can perform high-resolution imaging by driving a deformable mirror and a spatial phase modulator so as to correct a measured wavefront and imaging the fundus through them.
Most of the aberrations of the eye are lower-order aberrations, such as myopia, hyperopia, and astigmatism. However, the aberrations also include higher-order aberrations due to the fine recesses and projections on the optical system of the eye and the disturbance of a tear film. When the aberrations of the eye are to be expressed by a Zernike function system, most of the Zernike functions expressing the aberrations are Zernike second-order functions expressing myopia, hyperopia, and astigmatism. These functions slightly include Zernike third-order functions and Zernike fourth-order functions, and more slightly include higher-order functions such as Zernike fifth-order functions and Zernike sixth-order functions.
In general, the adaptive optics (AO) used in an ophthalmic apparatus models an aberration measured by the wavefront sensor with a function such as a Zernike function and calculates a correction amount for a wavefront correction unit by using the function. The amount quantitatively obtained by modeling an aberration with a function will be referred to as an amount of aberration. In addition, a wavefront-correction value with which the wavefront correction unit corrects an aberration by using the function will be referred to as a correction amount. In order to correct a complex shape, it is necessary to model an aberration with a function having many orders, calculate a correction amount, and control the wavefront correction unit.
If, however, a correction amount is calculated by modeling an aberration with a function having many orders, the calculation load becomes very heavy, and the calculation time increases, thus posing a serious problem. For the aberrations of the eye, in particular, it is very important to increase the processing speed because the state of tear and the state of dioptic adjustment always change and aberration correction needs to be repeated fast for the acquisition of a tomogram.